1. Field of the Invention
The invention relates to a printing or copying device having two transfer ribbons for the simultaneous printing of the verso and recto of a recording medium.
2. Description of the Related Art
Printing or copying devices of the type mentioned are known, for example, from U.S. Pat. Nos. 4,477,176 and 4,537,493. These relate to electrophotographic devices which have a photoconductor, on which at least two charge patterns can be arranged one after the other, and which have a transfer printing station with an associated single-sheet positioning device (turning device) which allows the electrophotographic printing device to be operated in two modes of operation, specifically in a first mode of operation, in which the applied toner images arranged one after the other on the charge pattern carrier are arranged one above the other or adjacently on one side of the single sheet, and a second mode of operation in which the sequence of the toner images located on the charge pattern carrier is arranged on the verso and recto of the single sheet. The fusing of the toner images on the single sheets takes place with the aid of a roller fuser station using pressure and high temperatures. In the mode of operation of duplex printing, in which a toner image is arranged on each side of the single sheet, the fusing of the verso and recto takes place simultaneously. For this purpose, the single sheet printed on one side has to be turned and printed again on the other side and it is then conveyed without contact to the roller fuser station, e.g. via an air cushion. This requires a very high mechanical expenditure if a high degree of operational reliability is to be guaranteed with a wide range of printing carriers.
A non-mechanical printing or copying device for multicolor and recto printing is known from European Patent application No. 0,154,695, which consists of a plurality of devices which are arranged one after the other and are operated simultaneously, a reversible deflection device for the paper web being arranged between the devices.
The connection of a plurality of complete printing systems one after the other is complex.
Furthermore, electrophotographic printing devices are known from U.S. Pat. No. 2,990,278 or Great Britain Patent application No. 2,040,226, in which charge patterns are produced character-dependently on a photoconductor with the aid of an exposure device and are fed to a developer station. The developed charge pattern is then lifted mechanically from the photoconductor by pressure with the aid of a ribbon-like transfer element and is transferred onto a recording medium. In order to be able to fuse the toner image on the recording medium, the toner image is heated on the, ribbon-like transfer element with the aid of a heating device, and the heated toner image is applied to the recording medium via a roller arrangement by pressure and heat. After the toner image has been transferred onto the recording medium, the intermediate carrier is cleaned from adhering toner in a cleaning station.
From the literature reference Patent Abstract of Japan, vol. 10, No. 300 (p 506); abstract of Japanese Patent application No. 61-117582, an electrophotographic single-sheet copying device for producing duplex printing is known. The copying device comprises a photoconductor drum on which verso and recto toner images are produced one after the other. Two transfer ribbons, which can be pivoted alternately against the photoconductor drum and are wrapped in each case around a heating roller, receive the verso and recto toner image. In this case, the heating rollers around which the transfer ribbons are wrapped are arranged so that they can be pressed onto one another, thus forming a transfer printing region. After the transfer of the toner images, the transfer ribbons are pivoted away from the photoconductor drum, and the toner images are simultaneously transferred onto a single sheet which is fed between the heating rollers.